1. Field of the Invention
The present invention generally relates to semiconductor chip manufacture and, more particularly, to selective etching of SiO.sub.2 to Si.sub.3 N.sub.4 using a stoichiometrically altered nitride etch stop.
2. Background Description
Sub 0.5 .mu.m Ultra Large Scale Integrated (ULSI) circuit applications require reactive ion etch (RIE) processes which will etch oxide highly selective to nitride. While this has been demonstrated on planar surfaces, stringent ground rules in advanced applications require high nitride selectivity over exposed corners. The most common example of this is where a diffusion contact overlaps a gate. Selectivities greater than 12 to 1 at the exposed corner of the gate are required to prevent leakage from the gate to the contact.
Selective oxide:nitride etch processes have been introduced, but these have required a delicate balance of polymer formation to ion energy control. These conditions are so sensitive to environmental fluctuations like nitride composition and uniformity that the process window becomes too small for manufacturing applications.
Thick nitride spacers on the edges of the device have been introduced to absorb any nitride erosion which may take place during the etch. This procedure, however, cannot be used in applications with very high packing densities.